As explained by Hans some time ago dividing the equi size by pi (3.1416...) will give the cube face size. In this case it works the other way round so this this 12870 comes from multiplying the cube face by pi.
It is a nice rule of thumb and thought I would write it down here again to clarify it for others (credit goes to Hans anyway)
--- In PanoToolsNG@yahoogroups.com, "Hans" <hans@...> wrote:
> 12870 pixels to be precise.
> A 5d MarkII with 15mm lens or a Nikon 16mp like the D7000 with the nikkor 10,5mm will give you enough.